#include <fstream>

int main()
{
   std::ifstream fin("in.txt");
   std::ofstream fout("out.txt");

   double x_0, y_0, x, y, x_last, y_last;
   fin >> x_0 >> y_0;

   x_last = x_0;
   y_last = y_0;
   
   double s = 0;

   while (fin >> x >> y)
   {
      s += (x - x_last) * (y + y_last);
      x_last = x;
      y_last = y;
   }
   s = abs(s + (x_0 - x) * (y_0 + y)) / 2;
   fout.precision(10);

   fout  << s << std::endl;

   fin.close();
   fout.close();

   return 0;
}